Such floating wind turbine assemblies are known from, for instance, WO 2015/048147 A1, WO 2009/064737 A1, KR 101488292, WO 2014/140653 A1, EP 2743170 A1, WO 2013/084632 A1, CN 102392796 A, US 2012/103244 A1, WO 2009/131826 A2 or US 2008/240864 A1.
In general, there are four categories of floater types to support floating wind turbines, each inspired by existing oil and gas concepts:    1. Semi-type    2. Spar/deep draft type    3. Barge type    4. Tension Leg Platform (TLP)
The types which can operate only in limited water depth, like articulated towers, are not listed above.
The main challenge of supporting a wind turbine, however, differs from supporting equipment topside, such as the case with drilling or mooring equipment. First of all, the device being supported, i.e. the turbine is slender and carries a significant mass at a large elevation above the floater keel, elevating the center of gravity of the whole object. This tends to make the floater unstable.
In operational conditions (when the turbine is producing power), a large thrust force is exerted at the nacelle elevation, i.e. high above the sea level. This will tend to give a significant overturning moment at the turbine connection to the floater.
All the concepts which have been patented so far are trying to limit the pitch/roll which is induced by this overturning moment, as it is the most demanding constraint imposed by turbine manufacturers.
The restoring in pitch/roll of a floater is proportional to its metacentric height, GM, which is the distance between the center of gravity G and its metacenter M. The higher the GM, the more stable the floater is.
For category 1, the semi-type, stability is achieved by elevating the metacenter M by putting water plane area far from the center of rotation. This will give in turn restoring on the pitch/roll degrees of freedom.
For category 2, spar or deep draft type, stability is achieved by lowering the center of gravity G. These structures also tend to lower the level of loading induced by waves by presenting small volumes close to the water surface.
For category 3, barge type, which are relatively large structures giving them large water plane area.
For category 4, TLP, there is a clear advantage in terms of motion performance and stability, as both heave, pitch and roll degrees of freedom are restrained. Stability in heave is obtained through putting a large buoyancy below the water surface and counteracting it with large tensions in the mooring system. For rotational stability, overturning moment is balanced by a difference in tensions in the legs. This may hold for a TLP with three groups of legs, but the same reasoning is valid with four or even more groups of legs.
An object of the present invention is to provide a floating wind turbine assembly, in particular of the TLP type, having improved stability, in particular when a large thrust force is exerted at the nacelle elevation.